RAM never affects FPS as such, it just chugs your PC along. It also depends whether you mean SPEED or SIZE.
If you're talking about low speed then it will have an affect on FPS (probably quite little, but load times will be noticably longer)
If it's small size, then you will get choppy gameplay as the RAM has to offload data to disk (Far slower than just using RAM). People may see this as a decrease in FPS, but tbh it doesn't really count.